The present invention relates generally to a cathode ray tube having an optically machine-readable coded marking on an external surface thereof and methods of producing CRTs (Cathode-Ray Tubes) having an, optically machine-readable coded marking, and more particularly to a CRT having a machine-readable marking on an optically machine-readable marking on a sidewall of its glass panel that is prevented from deteriorating during the process of manufacturing the CRT and a method of making the same.
The process of manufacturing CRTs generally fabricates those having different tube types sequentially or selectively using a common CRT assembly line. While CRTs of one type are being manufactured on the assembly line, one component part for use in that type is combined with others for use therein successively.
The common practice that has heretofore been followed is for assemblers to select proper parts to be assembled into CRTs by visually checking the markings attached to the respective parts or checking the parts themselves during the process of manufacturing CRTs. However, assemblers have experienced great difficulty keeping up with not only the work of selecting and conveying corresponding parts but also working meticulously as the size and resolving power of CRTs to be manufactured increase. For this reason, efforts are being made to automate the machining and assembling of parts the whole CRT production line including every step of assembly.
In order to implement the aforementioned automatic machining and assembly, assembly work should be carried out by distinguishing the type of CRTs being presently manufactured and properly selecting parts usable for CRTs of that type in addition to performing machining operations fit for them. In this sense, it has been arranged to attach a marking indicating a type of CRT to be manufactured on the sidewall of its glass panel during the manufacturing process.
In reference to the CRT having a machine-readable marking, particularly a bar code, on the sidewall of its glass panel, Japanese Utility Model Laid-Open No. 136465/1985, U.S. Pat. Nos. 4,374,451; 4,377,890 and 4,515,867, for instance, disclose CRTs of this sort. Of these CRTs, the one disclosed in Japanese Utility Model No. 136465/1985 (hereinafter called the former means) has a bar code printed on the sidewall of its glass panel using a heat-resistant material, the bar code having a number different from what is allocated to another code or a number to be incremented per code. Moreover, those disclosed in U.S. Pat. Nos. 4,374,451; 4,377,890 and 4,515,867 (hereinafter called the latter means) are similar to one another in that a double layer is formed on the sidewall of a glass panel by using two kinds of water glass coatings which differ in composition at the initial stage of the process of manufacturing a CRT before a bar code is formed by irradiating the layer with a laser beam in order to use the bar code for controlling the process of manufacture under computer control by machine-reading the bar code during the process thereof.
Since such a bar code is mainly used to record the state of a CRT being manufactured under control, it is formed on the sidewall of its glass panel at the initial stage of the process of manufacture. During the aforementioned process of manufacture, however, as the glass panel is treated by heating as well as with various kinds of acids after the bar code is formed, the bar code tends to deteriorate and may occasionally wear away.
For the reason stated above, a heat-resistant material has been used to form the bar code by printing, as in the case of the former means, or a laser beam has been used to print the bar code as in the latter means.
Notwithstanding, the former means still poses a problem unavoidably arising from the disappearance of the bar code thus formed, if worst, comes to the worst because the bar code formed on the glass panel deteriorates during the process of manufacturing the CRT.
On the other hand, though the latter means can at least prevent the bar code formed on the glass panel from deteriorating or wearing away during the process of manufacturing the CRT, the use of a laser beam for the formation of such a bar code not only renders the process of manufacture complicated but also makes the manufacturing system costly. The problem is that a unit cost of CRT tends to increase.